1. Field of the Invention
The present invention relates generally to Schottky barrier thin-film diodes. In particular, the present invention is a method for fabricating an amorphous silicon, thin film solar cell and Schottky barrier diode on a common flexible substrate.
2. Description of the Prior Art
Glow-discharge-produced, hydrogenated, amorphous silicon, thin-film, Schottky barrier diodes were first reported by Wronski et al. in an article entitled "Surface States and Barrier Heights of Metal-Amorphous Silicon Schottky Barriers" in Solid State Communications, Vol. 23, 1977, pp. 421-424. Schottky barriers were formed between Al, Ni, Cr, Pd, Au, Rh and Pt metal films and undoped amorphous silicon film. The undoped, hydrogenated, amorphous silicon (a-Si:H) was deposited from a DC discharge in silane onto a substrate held at 320.degree. C. to 350.degree. C. These substrates consisted of a three hundred angstrom thick phosphorus-doped a-Si layer on stainless steel which formed an ohmic or a low resistance contact to the undoped amorphous silicon film.
The study of metal-amorphous silicon Schottky barrier thin film diodes continues. In an article entitled "Silicide Formation in Pd-a-Si:H Schottky Barriers" in Appl. Phys. Lett., 39(3), Aug. 1, 1981, pp. 274-276, Thompson et al reported on amorphous silicon Schottky barrier thin film diodes with palladium. Palladium Schottky barriers were fabricated on quartz substrates. Cr or Ni was deposited to form a bottom electrode, followed by the subsequent deposition of a five hundred angstrom phosphorus doped n.sup.+ a-Si:H layer and an undoped a-Si:H layer having a thickness of 0.2 to 2.0 micrometers. They obtained a forward current density of 1.times.10.sup.-2 A/cm.sup.2 at one volt. This appears to be one of the highest forward current densities reported in the literature for metal-amorphous silicon barrier thin film diodes.
Thin film diodes are in widespread use. In general, thin film manufacturing techniques are less expensive and produce higher yields than wafer scale processing techniques used to fabricate crystalline or so-called "discrete" diodes. However, known thin film diodes have characteristics which are poorly suited for many applications. The rigid substrates on which they are fabricated prohibits their use in applications in which the device must be physically deformed. Contaminants from metallic contact layers often react with the semiconductor body during processing, degrading the diode's electrical characteristics. Since they are typically used with other semiconductor devices, the diodes must be separately fabricated and interconnected.
It is evident that there is a continuing need for improved, thin-film Schottky barrier diodes. A diode which can be fabricated using more cost effective, roll-to-roll processing techniques is desired. If a diode of this type were flexible it could be more readily shaped for applications in which planar devices are not suited. Production yields and diode characteristics can be improved if contaminants in the metal layer are isolated during fabrication. Schottky diodes having even larger forward current densities are of course desired. Diodes having these characteristics would be most useful if they could be efficiently fabricated on integrated circuits with other semiconductor devices.